Interstellar Medium
This set of notes by Nick Strobel covers: the interstellar medium--the effect
of dust, emission nebulae, 21 cm radiation, mapping galactic structure, and
molecules.These notes will be in outline form to aid in
distinguishing various concepts. As a way to condense the text down I'll
often use phrases instead of complete sentences. The vocabulary terms are
italicized.
Stuff between the stars. 10-15% of the visible mass of the Galaxy. 99% of the ISM mass is gas; 1% dust. ``So what?'' Why do we worry about the interstellar medium? The interstellar medium affects starlight and stars are
formed from ISM!
Dust-about the size of the wavelength blue light or smaller.
Water ice, graphite (Carbon), Silicon in highly flattened flakes or needles.
Effects of dust on light:
- Extinction--dimming of starlight at all wavelengths. In 1930 R.J.
Trumpler plots angular diameter of clusters vs. distance to cluster. Distance
found from inverse square law of brightness. IF clusters all have nearly same
linear diameter s,
then the angular diameter should equal a constant size / distance
(theta = s/D). But he
found a systematic increase of the linear size of the
clusters with distance. Unreasonable! It would mean that nature had put the
Sun at a special place where the size of the clusters was the smallest. More
reasonable: the Sun is in a typical spot. It's simply that more distant
clusters have more stuff between us and cluster so that they appear fainter
(farther away) than they really are. Extinction caused by scattering of light
out of the line-of-sight--less light reaches us.
- Reddening--extinction depends on wavelength: amount of extinction is approximately a constant
/ wavelength (E ~ 1/
).
Bluer wavelengths scattered more than redder wavelengths.
1/ behavior says that
the dust size must be about the wavelength of
light (on the order of 
cm). Less blue light reaches us so object appears redder than it should.
Trumpler showed that a given spectral type of star
becomes increasingly redder with distance. Same sort of process at work
to make the Sun appear redder when it is close to the horizon. Dust and gas
molecules in the air scatters out the bluer colors of sunlight from your
line-of-sight. When Sun close to horizon, this effect is enhanced (light is
going through more air).
About 90% Hydrogen, 10% Helium, and traces of other elements.
Observe Hydrogen in ionized, neutral atomic, and molecular forms. At visible
wavelengths dust has greater effect on light than gas. Looking at spectral
lines of binary, see narrow lines that do not move and other broader lines
shifting as stars orbit each other. The narrow lines are from gas in the ISM.
- H II Regions--fluorescence of hydrogen atoms. Ultraviolet light
from hot O & B stars absorbed by the Hydrogen gas and re-emitted mostly at
visible wavelengths, primarily 6563 Å(red color). Each UV photon produces a
visible photon. O & B stars only found in regions of star formation (know
why?). H II region spectra much simpler than star spectra-easier to decipher.
Stuff making stars is mostly Hydrogen and Helium. Distribution of H II regions
is in spiral pattern. O & B are spiral tracers also. The ``II'' of H II means
that Hydrogen is missing one electron. A He III nebula would be Helium gas with
two missing electrons. A H I nebula would be neutral atomic Hydrogen.
- 21 cm--emission line at wavelength 21 cm produced by neutral
atomic Hydrogen. In 1944 van de Hulst predicts it. Hydrogen in space is
in ground state. Electron moving around proton has spin parallel (same
direction) as proton or opposite (anti-parallel). Anti-parallel is slightly
lower energy state. Remember that atoms always want to be in the lowest energy
state possible. Sometimes Hydrogen atoms collide and the spins are re-aligned.
Eventually (on average few million years) the electron flips its spin to get in
lowest energy state. Low energy photon (frequency 1420.4 MHz which is at
wavelength 21.1 cm) emitted. This is a RARE transition, but there is lots of
Hydrogen in space! Enough to make noticeable amount of 21 cm radiation.
Milky Way (our galaxy) has about 3 billion solar masses of H I gas with about
70% of that further out in galaxy from the Sun. Most of the H I gas in disk is
located within 220 pc from the midplane of the disk. What's very nice is that
21 cm radiation is not blocked by dust!!
- a.
- Find density of atomic Hydrogen along line of sight from intensity of
21 cm line.
- b.
- Rotation curve--plot of rotation speed vs. distance from
Galaxy center. We assume that the gas clouds move in the plane of the disk
on circular orbits. Jan Oort 1927 finds stars closer to center complete
greater fraction of their orbit in a given time than stars farther out from
center--differential rotation (different angular speeds). Look at
Doppler velocities of Hydrogen gas along different lines of sight. Maximum
doppler velocity at distance = (solar distance) x sin(galactic longitude).
Observe maximum doppler velocity along different lines of sight to get rotation
curve.
- c.
- Map out Galaxy's structure. 21 cm line profile has several
Doppler-shifted peaks that are narrow and well-defined. IF the rotation curve
is already known, then we can use the doppler speed of peaks to get the
distance to the Hydrogen producing each peak. Use intensity of peak to get
density. Get spiral pattern in a thin disk for almost all of Galaxy! See
picture below:
- Other molecules:
, CO, 
OH, NH_3, SiO, 
, 100+ other
molecules, many of which have Carbon in them (organic). Most of the molecules
are and CO.
Molecular clouds:
- a.
- Most of the molecules in the ISM are clumped together into clouds
with masses anywhere from just a few solar masses to over a million solar
masses with radii ranging from a few pc to over 100 pc. Milky Way has about 2.5
billion solar masses of molecular gas with
about 70% of it in a ring at 4-8 kpc distance from the center. (The Sun is
about 8 kpc from the galactic center.) Not much molecular gas at 1-3 kpc
distance from center. About 15% of total molecular gas mass is located close
to galactic center within 1.5 kpc from the center. Most of the
gas is clumped in the spiral arms within the disk and stays within 120 pc of
the disk midplane. Stars form in the molecular clouds.
- b.
- absorption
lines detected in ultraviolet. However, gas and dust
become so thick in a molecular cloud that ultraviolet extinction is too large
to accurately measure all of the
in the interior of
the cloud.
Fortunately, we see evidence of a correlation between amount of CO and
so
we use the easily detected CO radio emission lines (at 2.6 and 1.3 mm) to infer
the amount of . CO
emission caused by
molecules colliding with CO molecules. More
means more collisions which means more CO emission.
- c.
- Is one gas cloud actually made of many smaller gas clouds? Some say
that 90% of locked
up in 5000 Giant Molecular Clouds with masses greater
than
and diameters greater than 20 pc with the monster ones
(diameters greater than 50 pc and having more than a million solar masses)
making up 50% of the total mass. Others say the giants are actually made of
smaller clouds.
The pictures come from various sites around the world. I have small ``thumbnail''
versions of the images on the page so the gallery page should load quickly.
Links are provided to larger-scale versions. If you want to see some gorgeous
pictures then go to the nebulae picture gallery. I
will be adding more pictures and explanations in the near future.
last updated 29 Nov 95
Nick Strobel --
Email:
strobel@astro.washington.edu
(206) 543-1979
University of Washington
Astronomy
Box 351580
Seattle, WA 98195-1580